Magnetic recording media generally comprise a magnetic coating provided on at least one side of a nonmagnetizable substrate. For particulate magnetic recording media, the magnetic coating comprises a magnetic pigment dispersed in a polymeric binder. The magnetic coating may also include other components such as lubricants; abrasives; thermal stabilizers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bacteriocides; surfactants; coating aids; nonmagnetic pigments; and the like.
Some forms of magnetic recording media, such as flexible magnetic recording tape, also have a backside coating applied to the other side of the nonmagnetizable substrate in order to improve the durability, conductivity, and tracking characteristics of the media. The backside coating typically comprises a polymeric binder, but may also include other components such as lubricants; abrasives; thermal stabilizers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bacteriocides; surfactants; coating aids; nonmagnetic pigments; and the like.
The magnetic coating and the backside coating, if any, of a majority of conventional magnetic recording media are derived from materials which require curing in order to provide magnetic recording media with appropriate physical and mechanical properties. To prepare such magnetic recording media, the uncured components of the magnetic coating or the backside coating, as appropriate, are dissolved in a suitable solvent and milled to provide a homogeneous dispersion. The resulting dispersion is then coated onto the nonmagnetizable substrate, after which the coating is dried, calendered if desired, and then cured.
Curing can be achieved in a variety of ways. According to one approach, the polymeric binder of the magnetic coating or the backside coating is derived from hydroxyl functional polymers which rely upon a chemical reaction between the hydroxyl functionality and an isocyanate crosslinking agent to achieve curing. The isocyanate crosslinking agent is typically added to the dispersion just prior to the time that the dispersion is coated onto the substrate.
One factor affecting the performance of magnetic recording media is the reactivity of the crosslinking agent. If the cure rate for a crosslinking agent is too slow, or if its level of cure is too low, then a coating incorporating the agent will tend to have poor green strength until the cure reaction has progressed sufficiently. As a result, the coating will be susceptible to damage, e.g., slit edge damage, during subsequent processing unless an inconvenient and expensive time delay is incorporated into the manufacturing process. Thus, to get a coating with good green strength, a faster reacting crosslinking agent is desirable.
Yet, the crosslinking agent must not react too fast. Once the crosslinking agent is added to a dispersion, the viscosity of the dispersion begins to gradually increase as crosslinking reactions take place. If these crosslinking reactions occur too quickly, the viscosity of the dispersion could increase so rapidly that it then would become extremely difficult to filter and coat the dispersion onto a nonmagnetizable substrate.
One commercially available crosslinking agent that has been used in magnetic recording media is commercially available under the tradename DESMODUR CB-601 from Miles, Inc. This product is an admixture of adducts formed by end-capping a blend of trimethylolpropane and 1,3-butane diol with toluenediisocyanate ("TDI"), an aromatic, sterically hindered diisocyanate. This admixture is relatively slow to react. Coatings prepared from this material tend to have a slow cure profile, poor green strength, and are susceptible to slit edge damage when substrates bearing such coatings are slit into appropriate widths to form magnetic recording tape.
Another commercially available crosslinking agent that has been used in magnetic recording media is available under the tradename PAPI from Dow Chemical Co. The PAPI material is an admixture of about 50% methylene bis(4-phenyl isocyanate) ("MDI") and about 50% of a blend of polymethylene polyphenyl isocyanates. This material reacts relatively quickly, but tends to yield a nonfilm-forming polyurea upon reacting with moisture. Inasmuch as moisture is a prominent reactant in the cure reactions of magnetic and backside coatings, we believe that the nonfilm-forming nature of such materials tends to reduce the resilience, toughness, and the like of coatings incorporating such materials.